or bison, or antlr. Or just a parser you would write yourself. It really depends on how complete you want it, i.e. do you want * and / to be prioritary over + and -, do you want the ability to enter parentheses, etc.

Yes, I'll prefer parser if it created myself, but I am beginner in c++. I prefer transform available code to my needed. Can't write code from scratch.
It seems that there are no so simple calculators and I will write my.
Without parentheses, it's simple.